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Conceal Control Horns And Pushrods

This article was originally published in Fly RC’s April 2016 issue.
By Gary A. Ritchie

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Figure 1. The beautiful lines of this RC scale Nakajima KI-84 are spoiled by the protruding aileron pushrod and control horn (white arrow).

In the world of RC scale model airplane building, one of the trickiest problems is how to “hide” external pushrods and control horns inside wing surfaces. With tail control surfaces these devices can normally be concealed within the fuselage. But on wings, which are often very thin, the problem is far more difficult.
Take a look at Figures 1 and 2. In Figure 1, an RC model Nakajima Ki-84 fighter, the aileron control horn and pushrod protrude from the lower wing, spoiling the scale appearance of the airplane. The model Spitfire Mk VB (Figure 2) suffers from the same problem.

But look now at the underside of my P-40 Kitt yhawk (Figure 3) and my Ilyushin Il-2M3 Sturmovik (Figure 4). No pushrods, no control horns – the wings are whistle clean and very scale-like, while the control surfaces move from within as if by magic. So, how to do it?

I know of two different methods of achieving this. The first method is simple, elegant and inexpensive, but it offers only a limited amount of control throw. So it is oft en used on ailerons. The second way is more costly, but it provides as much control throw as you need and, as such, works well with flaps. Let’s begin with the first method.

MIKE MACFARLAND METHOD

This method was reported in RC-Sport Flyer magazine a few years ago by my colleague Mike MacFarland. For each control surface you will need a small, ? at servo such as a Hitec HS-82MG, a large DuBro EZ-connect and a 1/2 inch long section of 1/8-inch diameter brass tubing. You will also need to cut a 3 inch long section of 3/32-inch diameter wire.

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Figure 2. Look how the graceful wings of this Spitfire Mk VB are cluttered up by the obvious non-scale protruding control horns and pushrods seen here below the wings.

The installation is shown in Figure 5. On the left side of the photo is the HS-82MG servo mounted sideways in a plywood- reinforced rib. The brass tube was slipped through a 1/8-inch hole that was drilled through the EZ- Connect and then the EZ-Connect and the brass tube assembly were mounted on the servo horn.

The aileron is on the right side of the photograph. The 3/32- inch diameter wire has been bent at a 90° angle about 1 inch from one end and glued securely to the aileron with its long end protruding through the leading edge (Figure 6). In Figure 7, the leading edge of the aileron is pressed flush against the trailing edge of the wing with the metal wire protruding through the hole in the trailing edge where it slides through the brass tubing mounted on the servo horn.

Here is how it works. When the servo horn moves upward it pushes the end of the metal wire upward, while it slides back and forth through the brass tube. This causes the aileron to move downward and vice versa. This elegant litt le device is inexpensive and simple to build. Its only drawback is that the range of motion is limited by the thickness of the wing. With an aileron, this is normally not a problem because in scale models the control throw on ailerons is typically not very great. With flaps however, where up to 45° of throw may be needed, this device may not be suitable.

ROTARY AILERON LINKAGE METHOD 

I was made aware of this method many years ago by a friend and fellow modeler named Kelvin Ritchie (no relation). This device is o? ered by Walt Dimick of I.F.R. Machine Works (www.irfmachineworks. com). It consists of a nylon/metal coupler that screws securely to the servo spline shaft , a long bent hardened drive shaft and a ? at “precision pocket” (Figure 8) into which the end of the driveshaft slips. The servo with the rotary linkage is mounted on its side inside the wing (Figure 9), while the precision pocket is mounted inside the control surface (Figure 10). The ? nished installation, without aileron, can be seen in Figure 11.

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Figure 3. Pushrods and control horns are hidden on both ailerons and flaps of my Ilyushin IL-2M3 Sturmovik.
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Figure 4. The belly of my P-40 Kittyhawk is as clean as a whistle owing to the concealed control horns and pushrods on both flaps and ailerons.
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Figure 5. This is a photo of a wing mock-up. At the left you can see the servo mounted sideways within the wing rib. The brass tubing has been inserted through a 1/8-inch hole pre-drilled into the EZ-connect. The aileron is shown on the right. The 3/32-inch metal bar, which has been bent at a 90° angle and glued to the inside of the leading edge, protrudes about 2 inches through a hole in the front of the aileron’s leading edge.
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Figure 6. This is the same mock-up as shown in Figure 5, but viewed from a different angle. This view shows the manner in which the metal bar is fastened to the inside leading edge of the aileron and protrudes through the aileron towards a hole in the trailing edge of the wing.
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Figure 7. In this photo the aileron is pressed against the trailing edge of the wing so that the metal bar goes through the trailing edge and engages the brass tubing. As the servo horn moves up and down it causes the metal bar to pivot down and up causing the aileron also to move down and up.
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Figure 8. This shows the components of the rotary aileron linkage: (A) a nylon/metal coupler is screwed to the servo spline shaft; (B) a bent hardened drive shaft slips into this coupler and is fastened securely in place with set screws. This shaft then protrudes through the trailing edge of the wing into the leading edge of the control surface, which contains the precision plate (C).
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Figure 9. The servo, coupler and driveshaft are mounted lying flat inside the wing.
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Figure 10. The precision plate is glued securely just behind the leading edge of the control surface.
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Figure 11. This shows the location of the rotary aileron linkage mechanisms inside the wing of my Sturmovik before the wing was planked.
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Figure 12. The final installation of the rotary aileron linkage mechanism in the wing of my P-40.
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Figure 13. My DeHavilland Mosquito night fighter is sleek and clean with no ugly external control mechanisms showing.

When the servo is actuated the end of the driveshaft , which is inside the precision pocket, rolls from side to side causing the control surface to move up and down. The design of this device provides a great deal of control throw.

What I oft en do is to use the inexpensive MacFarland method to drive my ailerons and the pricier rotary aileron linkages for my flaps. I have used these two devices on several of my scale ships, including the P-40 and the Il-2M3 shown in my photos, as well as a Mosquito night fighter that I reported on in RC-Sport Flyer in December 2009 (Figure 13). I find both methods to be highly reliable and would encourage you to try them out.

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